A: Generally speaking, when referring to the sun, the definition is when the leading or trailing edge of
the sun passes the horizon. This time is somewhat different from the more
specific definition which is called the geometric rise or set. The
geometric rise or set of a celestial body is when the center of the object, such as a star,
passes the horizon and there is no atmospheric refraction.

Please see the next few questions.

Q:I see that March 18 has exactly 12 hours of daylight. I thought that
the equinox was March 21 (or September 22).What is the exact definition of
the equinox and when does it occur?

A: The dictionary definition is: either of the two times,
the vernal equinox (about March 21) or the autumnal equinox (about September 22),
during a year when the sun crosses the celestial equator and when
the length of day and night are approximately equal.

That definition is not exactly correct.
The astronomy book, "Stars and Planets" by Jay M. Pasachoff and Donald H. Menzel, from Peterson
Field Guides, ISBN 0-395-53759-2, further explains the equinoxes:

"Although day and night are theoretically equal in length on the days of the
equinoxes, that would be true only if the sun were a point, not a disk, and
if the earth's atmosphere did not bend sunlight. However, the top of the
sun actually rises a few minutes before the center of the sun's disk, the
point used in calculations. Also, the earth's atmosphere bends sunlight, so
we can see the sun for several minutes before the time sunrise would occur
and after the time sunset would occur if the earth had no atmosphere."

The algorithms used on this site take the atmospheric refraction and the diameter of
the sun into account so the times should be accurate to the leading or
trailing edge passing the horizon.

See also the next two questions.

Q: Why doesn't the site show seconds? What day has exactly 12 hours of sunlight?

A: This site calculates the times using your latitude and longitude,
these should be fairly accurate if using a predefined location,
but it is not necessary to be extraordinarily accurate—numbers within
a few minutes of both latitude and longitude (or a couple of miles or kilometers in distance) are sufficient.
The calculations do not take into account your elevation, which if very high, can affect the times
by a couple of minutes. For most locations, which are at or near sea level, this is not a concern,
but what does affect all locations on Earth is the atmosphere. It causes
atmospheric refraction which makes predicting times to within a
second impractical. This is why we cannot say when a day has exactly 12 hours of sunlight.
Also see the next question.

Q: What is atmospheric refraction and how does it affect times calculated on this site?

Here on Earth we have an atmosphere which actually
bends light down toward the surface when the object is near the horizon,
allowing us to see the sun (and the moon, planets and stars) before and after they would
normally be visible if there were no atmosphere. This is called
atmospheric refraction.

The amount of 'bending',
called the refractive index, varies
due to many factors and is not constant
from day to day or even by the minute. The two main factors that affect the refractive index are
the amount of water vapor in the atmosphere and the temperature of the atmosphere. A third, smaller
factor is the atmospheric pressure and then there are many much smaller factors, such as the presence
and amount dust in the air. All these factors together will affect the refractive index and when taking
into account that we are interested in sunrise and sunset, when the light goes through much more atmosphere
than at noon, we realize that the refractive index will have quite an effect on the times, making them off
by many seconds. Because these factors are always changing it makes no sense to claim that the times
are accurate to a second. It is also now apparent that we cannot say that any specific day has
exactly 12 hours (or some other number) of sunlight.

At this point, you may also be interested in doing internet searches on
scintillation (twinkling of stars)
and mirage.

Q: What elevation is used to calculate the times?

A: The elevation is always calculated at sea level. Mountains and
other large landmarks may also affect local times, these are not accounted for
in the times presented on this web site or the Windows Sunrise Sunset Calculator.
Although you may not see the sun at a certain time, say sunrise, because of a landmark, or you may see
it sooner due to your being at a higher elevation, the overall sky will still be lit up at the
given sunrise time. It may may be easier to understand this if you think of a sunrise time such as
civil twilight. No one expects to see the sun when it is 6 degrees below the horizon, unless you
are many miles high above the surface of the earth, but at the given twilight time the sun will
light up the sky no matter if you are at sea level or on the summit of a mountain.

Q: What is the difference between "solar noon" and "directly overhead?"

A: Solar noon is the time when the sun appears highest in the sky (toward the north or south pole) for that location.
This may or may not be "directly overhead", i.e. perfectly straight up from your location.
This is sometimes called "directly overhead", but this is incorrect unless your latitude is between
the Tropic of Cancer (23 26' 22" N) and the Tropic of Capricorn (23 26' 22" S) and the earth's position is such that
the sun is straight above the observer. For latitudes higher than those,
the sun will never get "directly overhead."

Q: What are some places that have interesting sunrise and sunset times?

A: If you have made it to this question, you have probably found that
the North Pole and the South Pole are not very interesting for sunrise and sunset times.
There are two reasons for this: 1) at the poles, the sun is usually either always up or
always down; 2) all the time zones converge, thus making selection of a time zone meaningless.

McMurdo Station on Antarctica.
McMurdo, at about 78° 51' S, uses the same time zone and
DST rules as New Zealand.
That time is used because most of the flights transporting people and some supplies to the base originate from Christchurch, New Zealand.

Alert, Nunavut, Canada,
at about 82° 30' N, is the northernmost permanently inhabited place in the world.
It is in the Eastern time zone and observes North America
DST rules.

In the 48 contiguous United States, not including islands:

Lubec, Maine is the easternmost
incorporated place. During winter, it has the earliest sunrises.

The westernmost point is Cape Alava, Washington.
Nearby is Cape Flattery, Washington, which is almost as far west. During summer, they will have
the latest sunsets—Cape Flattery may have the later sunsets by a few seconds due to it being more northern.

During winter, places such as
Cape Blanco in Oregon
and multiple places in California, such as
Cape Mendocino, Point Cabrillo Light Station, Point Arena Lighthouse, Point Sur Lightstation and Point Arguello Lighthouse may have the latest sunsets.
While not as far west, they each may have later times due to being farther south.

Q: Solar noon drifts around in a funny fashion. Also, in the northern hemisphere, some cities have the earliest sunset in early December and
the latest sunrise in early January instead of some day near the winter solstice. Why?

A: Most of this is probably explained by what is called "analemma."
That is:

The Earth is tilted on its axis 23.5° in relation to the plane of its orbit around the sun.

The Earth does not orbit the sun in a circle, but in an ellipse.

The Earth's orbit being elliptical brings up a point that will not be intuitive:
The Earth is closest to the sun in early January—that is during
winter in the northern hemisphere. It is farthest from the sun in early July.
(See Apsis - The perihelion and aphelion of the Earth.)
Being closer to the sun has nothing to do with how cold it is in a certain hemisphere. The tilt of the Earth
is what gives a place more or less sunlight during the day, thus making that area warmer or cooler.

Also note that the tilt is what makes the solstices the days
they are and that the Earth is not closest or furthest away from the sun on these days.

The speed of the Earth in its orbit will be greatest when it is closest,
thus in January the changes in sunrise and sunset times may be affected more than expected.
Its speed will be slowest in July and times may not change as much.